High energy pyrophoric fuel compositions



United States Patent O U.S. Cl. 14922 11 Claims ABSTRACT OF THE DISCLOSURE A high energy pyrophoric fuel capable of spontaneous ignition consisting essentially of from about 25 to about 95 weight percent of an organoaluminum compound and from about to about 75 weight percent of a boron hydride. A method of effecting combustion comprises contacting said fuel with air or other oxidizer.

BACKGROUND OF THE INVENTION Fuels presently available are generally hydrocarbon fuels. However, these fuels generally do not possess spontaneous ignition characteristics. Furthermore, since such fuels normally are not pyrophoric, they require an igniter system to effect their ignition. Thus, the ignition delay of a hydrocarbon fuel depends not only on the inherent ignition delay of the fuel itself, but also on any mechanical delay inherent in the ignition system. Because of the presence of an ignition system, the possibility of failure is increased. In contrast, the fuels of this invention provide fuels which are pyrophoric and, therefore, will ignite on contact with an oxidizing agent, thus eliminating any need for an igniter system and the disadvantages associated therewith. The fuels herein described generally possess higher energies than the hydrocarbon fuels.

Fuels based on organometallic compounds have undergone considerable research and their advantages have been recognized in the art. For example, the Bauerle et al. Patent, U.S. 3,127,735, deals with reduced ignition-delay fuels prepared from organometallic compounds and organoboranes. More specifically, said patent discloses fuels (propellants) composed of a variety of organometallic compounds, including aluminum alkyls and alkyl boron compounds, such as trialkyl boranes, alkyl boron halides and alkyl diboranes. To function as a fuel, the propellant must react with a liquid oxidizer such as liquid oxygen, nitrogen tetroxide, hydrogen peroxide, liquid fluorine, fuming nitric acid, and the like.

The fuels of this invention vary substantially from the fuels described in the above-mentioned Bauerle et al. patent. In the instant invention, the fuels comprise an organoaluminum compound and boron hydrides having from four to ten boron atoms, such as tetraborane, pentaborane, and decaborane. Another important feature of this invention is that the fuels do not require a strong oxidizing agent, such as liquid oxygen, hydrogen peroxide and the like, but are fully operational when air is used as the oxidizer, even at such low pressures as 0.25 atmosphere.

SUMMARY OF THE INVENTION In essence, this invention provides fuels capable of spontaneous ignition which comprise an organoaluminum compound and a borohydride having four to ten boron atoms. This invention also provides a method of effecting spontaneous ignition by reacting said fuel with an oxidizer.

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DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention comprises the discovery of fuels with extremely short ignition delays. These fuels can be provided by a composition consisting essentially of from about 25 to about 95 weight percent of an organoaluminum compound as defined below, and from about 5 to about weight percent of a boron hydride having from 4 to about 10 boron atoms.

The preferred fuels comprise substantially from about 45 to about weight percent of one or more organoaluminum compounds and from about 5 to about 75 weight percent of one or more boron hydrides. A most preferred embodiment of this invention consists essentially of from about 55 to about 65 weight percent of one or more alkyl aluminum compounds and from about 35 to about 45 weight percent of one or more boron hydrides.

When contacted with an oxidizer, the fuels of this invention ignite spontaneously. By spontaneously, it is meant that the ignition takes place within one second, and preferably, within a much shorter interval of time. The instant fuels ignite generally within about 25 milliseconds. However, the ignition delay time for the preferred compositions of this invention is from about 4 to about 10 milliseconds. When the recorded ignition delay time is corrected for the discrepancy inherent in the recorder, as explained below, the true ignition delay of the most preferred compositions drop to about 0.5 millisecond after contact.

A prime component of the fuels of this invention is an organoaluminum compound having the empirical formula R Al wherein R groups are selected from hydrocarbon radicals such as alkyl groups having up to about 4 carbon atoms or aryl, aralkyl and alkaryl groups having up to about 8 carbon atoms, hydrogen, and halogens, especially chlorine, bromine and fluorine. It should be noted, however, that the organoaluminum compounds should not have more than one aryl, alkaryl or aralkyl group. Although in some instances iodine and more than one aromatic groups may be present in the organoaluminum compounds, generally, such compounds are not very pyrophoric and, therefore, preferably should not be employed in the fuels of this invention.

A further requirement with regard to organoaluminum compounds is that the compounds should have at least one hydrocarbon group present. In other words, any given aluminum compound cannot have more than two hydrogen and/or halogen atoms present. The hydrocarbon groups which are bonded to aluminum may be halogenated with chlorine, bromine, and fluorine, and in limited instances, also with iodine. The three R groups bonded to aluminum may be the same or different, keeping in mind the above noted requirements.

The alkyl groups attached to aluminum may be either straight chain or branched chain containing preferably up to four carbon atoms. Non-limiting examples of alkyl aluminum compounds, that is compounds whose hydrocarbon groups are selected only from alkyl radicals, are trimethyl aluminum, triethyl aluminum, tri-n-propyl aluminum, tri-n-butyl aluminum, triisobutyl aluminum, triisopropyl aluminum, diisobutyl aluminum hydride, diethyl aluminum hydride, di-n-propyl aluminum hydride, diethyl aluminum chloride, ethyl aluminum dichloride, methyl aluminum sesquichloride, ethyl aluminum sesquichloride, diisobutyl aluminum chloride, methyl aluminum sesquibromide, diethyl aluminum iodide, methyl diethyl aluminum, ethyl di-n-butyl aluminum, dimethyl isobutyl aluminum, diethyl methyl aluminum, and the like. It should be noted that some organoaluminum compounds exist as dimers and trimers. For example, trimethyl aluminum, triethyl aluminum, and tri-n-isopropyl aluminum exist as dimers, while dialkyl aluminum hydrides exist as trimers.

The organoaluminum compounds useful in the fuels of this invention may contain one aryl, aralkyl, or alkaryl group. Thus, a phenyl group may be attached to aluminum producing such compounds as dimethyl phenyl aluminum, diethylphenyl aluminum, diisopropylphenyl aluminum, ethylphenyl aluminum hydride, and the like. Although polyaryl and alkaryl aluminum compounds could also be employed, such compounds have low pyrophoric properties and, therefore, the resulting fuels would have less desirable ignition properties.

The alkaryl groups which may be attached to aluminum contain a single aromatic ring with from 1 to 3 lower alkyl substituents such as methyl, ethyl, and propyl. Non-limiting examples are dimethyltolyl aluminum, diethyltolyl aluminum, dimethyl(2,4-diethylphenyl)aluminum, (3-isopropylphenyl)methyl aluminum hydride, (4-chloro-2-methylphenyl)methyl aluminum hydride, and the like.

The aralkyl groups which may be attached to aluminum may contain up to three carbon atoms in the alkyl chain and a single aromatic ring substituted with one to three lower alkyl substituents. Non-limiting examples are benzyldimethyl aluminum, benzyldiethyl aluminum, dimethyl(4- methylbenzyl)aluminum, (4 chloro 2 isopropylbenzyl) ethyl aluminum hydride, methyl(2,3,4-trimethylbenzyl) aluminum hydride, [3-(4-isopropylphenyl)propyl]methylaluminum hydride, and the like.

As mentioned above, the fuels of this invention may contain a mixture of different aluminum compounds, as for example, a mixture of two different trialkyl aluminums or a mixture of a trialkyl aluminum, a dialkyl aluminum hydride, and/ or dialkyl aluminum halide. In general, when two different organoaluminum compounds are present, they do not remain merely as a physical mixture. Under such conditions, molecular rearrangement or disproportionation usually takes place. This means that the various organic groups, hydrogen and/or halogen radicals, interchange until dynamic equilibrium is reached. The end result is that the composition is not merely a mixture of two or more compounds, but a mixture of many compounds containing compositions intermediate between the original compounds.

The operation of disproportionation may be illustrated by the following example. If originally triethyl aluminum, triisobutyl aluminum, and diethyl aluminum hydride are mixed, the resulting disproportionation product will contain the following compounds: diethyl isobutyl aluminum, ethyl isobutyl aluminum hydride, ethyl diisobutyl aluminum, and diisobutyl aluminum hydride, in addition to the original compounds. If such compounds as dialkyl aluminum halide, R AIX, and dialkyl aluminum hydride, R AlH, are mixed, the redistribution product would probably contain, in addition to the original compounds, the following intermediate compounds: RAIX RAlH- and RAlXH.

The second ingredient of the fuels of this invention are boron hydrides having from four to about ten boron atoms. Illustrative examples of boron hydrides are tetraborane, B H pentaboranes, B H and B H 1; hexaborane, B H and decaborane, B H

The important feature of this invention is that the ignition delay, defined as the time elapsed from the moment of contact of the fuel with an oxidizer to the moment of ignition, is lower than the predicted ignition delay. The predicted ignition delay values for two component fuels were obtained by plotting on a graph (where one axis represents the ignition delay time While the other represents the' fuel composition) the ignition delay for each component of the fuel. Thus, one end of the composition axis represents 100 percent of one component while the other end represents 100 percent of the other component. The line in between represents fuel compositions containing a certain amount of both components. When a straight line 'lriethylaluminum: 0

is drawn between the two points representing the ignition delays of the pure components, one can obtain from the graph a predicted interpolated ignition delay value depending on the relative amounts of the two components.

The ignition delay determinations were measured by noting the time between the contacting of the fuel with an oxidizer and the time of ignition. This was accomplished in an apparatus comprising a glass combustion tube, a solenoid operated injector, and timing circuits. When the starter was depressed, the solenoid activated a fuel ejector injecting a sample of the fuel into the combustion tube and, at the same time, activating an electronic timer. When the fuel ignited, the light was detected by a photocell which stopped the timer. It should be noted that the above-described apparatus did not give absolute ignition delay time but only relative values. When the ignition delay was studied by a high-speed motion picture camera, it was determined that in order to obtain absolute ignition delay times, about 3.5 to 4.5 milliseconds should be subtracted from the ignition delay values obtained by the above-described apparatus.

Table I below illustrates the ignition delay characteristics of the fuels of this invention. In the table, the actual (uncorrected) and the predicted interpolated ignition delay values are reported for the indicated compositions.

TABLE I Actual ignition delay in milliseconds Fuel components in weight percent Pentaborane In the above table, the predicted ignition delay values are indicated as being greater than the shown values. The reason for this is that pentaborane is not pyrophoric at temperatures below 77 F. Since the tests were conducted below 77 F., the ignition delay for pentaborane was infinite. Measurable ignition delay time was obtained by adding 10 volume percent of an aluminum alkyl to pentaborane. It, therefore, follows that since the material contained an aluminum alkyl, the ignition delay was much smaller than if it were pure pentaborane.

From the above table it is seen that a great reduction in the ignition delay characteristics of aluminum alkyls is obtained when a boron hydride, such as pentaborane, is admixed with an aluminum compound. This result is most unexpected and startling because boron hydrides employed in this invention per se are pyrophoric only at temperatures above 72 F. Thus, it was found that by adding a non-pyrophoric material to a pyrophoric material having a considerable ignition delay, a fuel having greatly reduced ignition delay is obtained.

Table II below contains additional fuel compositions and their ignition delay values obtained in the same manner as described above.

TABLE II Ignition delay in Weight millipercent seconds Fuel composition:

Triethyl aluminum Diethyl aluminum hydride 5 Pentaborane Triethyl aluminum Pehtaboranai Dec-aborane Ti'iethyl aluminum Diethyl aluminum hydride Pentaborane.'.

Pentaborane. L

Triethyl aluminum Trimethyl aluminum.- Pentaborane. 'IVriethyl aluminum... Diethyl aluminum hydr Bentaboraue Triethyl aluminum Diethyl aluminum hydride. Trimethyl aluminum... 8 Pentaborane. 5 Trimethyl aluminum. 8 Dlethyl aluminum hydr 7 7 Pentaborane g Decaborane Pentahorane Additional examples of fuels with short ignition delays which are Within the scope of this invention are listed below.

Fuel composition: Weight percent Triisobutyl aluminum 50 Diethyl aluminum hydride 45 Pentaborane 5 Diethyl methyl aluminum 50 Diethyl aluminum chloride 30 Decaborane 20 Ethyl aluminum sesquichloride 75 Decaborane 25 Tri-n-butyl aluminum 40 Tetraborane 50 Decaborane 10 Diethyl aluminum iodide 80 Hexabo'rane 20 Propyl aluminum dichloride 70 Tetraborane Pentaborane 15 Dimethylphenyl aluminum 60 Pentabdrane 30 Decaborane 10 Ethylmethylphenyl aluminum 55 Hexaborane 40 Decaborane 5 Diethylphenyl aluminum 40 Pentaborane 15 Ethylphenyl aluminum hydride 45 Pentaborane 55 (2,4-diethylphenyl)methyl aluminum hydride 50 Triethyl aluminum 25 Pentaborane 25 Hexaborane 10 (4-ch1oro-2-methylphenyl)dimethyl aluminum 35 Triethyl aluminum 45 Tetraborane Benzyldiethyl aluminum 70 Pentaborane 30 Dimethyl(2,3,4trimethylbenzyl)aluminum 48 Dimethyl aluminum hydride 40 Pentaborane 5 Decaborane 7 6 Fuel composition: Weight percent (4 chloro-2-isopropylbenzyl)methyl aluminum hydride 10 Methyl aluminum sesquichloride 30 Triethyl aluminum 25 Pentaborane 25 Decaborane '10 The fuels of this invention are pyrophoric. This is one of the important features and advantages of the instant fuels because it makes it possible for the fuels to be employed without any special oxidizer as long as there is air. A further feature of this invention is that the fuels are operable under conditions where the atmospheric pressure of air is as low as 0.5 atmosphere, or even 0.25 atmosphere. Although the instant fuels operate satisfactorily in air containing any amount of water vapor, superior performance is obtained when air contains at least 0.00020 pound of water per cubic foot of air at atmospheric pressure.

In addition to air, other liquid or gaseous oxidizers may be used. Other oxidizers which may be also employed in combination with the fuels of this invention include oxygen, nitrogen tetroxide, hydrogen peroxide, chlorine trifiuorine, bromine pentafluorine, white fuming nitric acid, red fuming nitric acid, liquid fluorine, liquid fluorine and liquid oxygen mixtures, oxyhalides, nitrogen fluoride as well as other oxidizers known to those skilled in the art.

With any of the above-mentioned oxidizing agents, combustion may be either of the external or internal types. That is, combustion may take place on the external surface of a vehicle, or in a motor, such as a ramjet or a rocket. When air is employed as the oxidizing agent, combustion preferably takes place on the external surface. However, when other gaseous or liquid oxidizers are employed, then an internal type combustion is preferred.

The fuels of this invention generally may be employed in all applications Where a high energy propellant is required. For example, the instant fuels may be used as pyrophorie rocket propellants. The oxidizer may be either air or other oxidizing agent, depending Whether the rocket is operated within the atmosphere or outside. The fuels may also be used advantageously in turbine engines to re-ignite the fuel normally employed in case of a flameout. Another important application for these fuels is as missile outboard ejector fuels. Such fuels must possess an extremely short ignition delay time and, for this reason, the fuels of this invention are particularly suitable for this application.

As stated above, organoaluminum compounds undergo disproportionation when two or more such compounds are mixed. However, for simplicity, the compositions of this invention claimed and exemplified herein are not defined in terms of redistribution products, but only in terms of the organoaluminum compounds added to form the composition. For practical reasons, the instant compositions contain one to three added organoaluminum compounds and from one to three boron hydrides, as exemplified above. However, these compositions may also contain four, five, or even nine organoaluminum compounds, and similarly, more than three boron hydrides.

Having fully disclosed the novel fuels of this invention and the method of effecting a spontaneous ignition, it is desired that this invention be limited only within the lawful scope of the appended claims.

We claim:

1. A high energy pyrophoric fuel capable of spontaneous ignition, said fuel consisting essentially of (i) from about 25 to about weight percent of an organoaluminum compound having the formula R Al, wherein the R groups are selected from (a) hydrocarbon radicals selected from the group consisting of alkyl groups having up to about 4 carbon atoms, and aryl, alkaryl, and aralkyl groups having up to about 8 carbon atoms, such that no more than one aryl, alkaryl and aralkyl group is present, and

(b) hydrogen, chlorine, bromine, and fluorine, such that at least one R group is a hydrocarbon radical, and

(ii) from about to about 75 weight percent of a boron hydride having from 4 to about 10 boron atoms.

2. A fuel composition of claim 1 wherein said organoaluminum is an alkyl aluminum compound.

3. A fuel composition of claim 2 wherein the amount of said alkyl aluminum compound is from about 45 to about 80 weight percent and the amount of said boron hydride is from about 20 to about 55 weight percent.

4. A fuel composition of claim 2 wherein the amount of said alkyl aluminum compound is from about 55 to about 65 weight percent and the amount of said boron hydride is from about 35 to about 45 weight percent.

5. A fuel composition of claim 3 wherein said alkyl aluminum is selected from the group consisting of trialkyl aluminum and dialkyl aluminum hydride.

6. A fuel composition of claim 3 consisting essentially of about 57 weight percent triethyl aluminum and about 43 weight percent pentaborane.

7. A fuel composition of claim 3 consisting essentially of about 14 weight percent triethyl aluminum, about 42 weight percent diethyl aluminum hydride, and about 44 weight percent pentaborane.

8. A fuel composition of claim 3 consisting essentially of about 56 weight percent triethyl aluminum, about 31 weight percent pentaborane, and about 13 weight percent decaborane.

9. A fuel composition of claim 3 consisting essentially of about 14 weight percent triethylaluminum, about 41 weight percent die'thylaluminum hydride, about 32 weight percent pentaborane, and about 13 weight percent decaborane.

10. A fuel composition of claim 3 consisting essentially of about 29 weight perccent triethyl aluminum, about 27 weight percent trimethyl aluminum, and about 44 weight percent pentaborane.

11. A fuel composition of claim 3 consisting essentially of about 27 weight percent diethylaluminum hydride, about 28 weight percent trimethyl aluminum, and about 45 weight percent pentaborane.

References Cited UNITED STATES PATENTS 2,935,839 5/1960 Beatty et al. -213 X 3,057,763 10/1962 Hunt et al. 149-22 3,062,856 11/1962 DAlelio 149-22 X 3,127,735 4/1964 Bauerle et al. 14922 X 3,177,652 4/1965 Lewis 14922 X CARL D. QUARFORTH, Primary Examiner M. J. SCOLNICK, Assistant Examiner US. Cl. X.R. 149-109 mgr UNITED STATES PATENT orrrcr CERTIFICATE OF CORRECTION Patent No. 9 59 Dated March 5 97 Inventr s Martin E, Gluckstein and Duane C. Hargis It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 36, for read 12 Line 37, for "2" 1 read 6 5 Line 58, for "6 read 5 Column 5, line 60, for Pentaborane l5" read Pentaborane 6O Line 65, for "Pentaborane read Pentaborane l5 Column 5, lines 57-75, read the following groups of lines as indicating individual fuel compositions: Lines 57-59; -42; 5 -47; 9; 5 -52; 53-55; 5 -5 59- 3- 67-69; 70-71; 72-75. Column 6, lines 2-7 should be read as one fuel composition.

Arrears saw-emfiiwssalr. wmrm r. as A mg flowisaioner of We qg gg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 9 59 Dated March 97 m Martin E, Gluckstein and Duane C. Hargis It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

"I Column 4, line 36, for read l2 3 Line 37, for '2" read 6 Line 38, for "6" read 5 Column 5, line 60, for "Pentaborane l5" read Pentaborane 6O Line 65, for "Pentaborane read Pe ntaborane l5 Column 5, lines 37-75, read the following groups of lines as indicating individual fuel compositions: Lines 37-39; 40-42; 5 +5 +7; 9; 5 -52; 53-55; 5 -5 59- 3- 67-69; 70-71; 72-75. Column 6, lines 2-? should be read as one fuel composition.

SPGll-D AND SEALED ms 1 815% (SEAL) Attest:

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